What actually happens in a synchronous generator when power system is subjected to any kind of disturbance (like any kind of fault, Heavy load is switched on, continuous load change in grid, lightening)? I know that the "system is oscillating" and I am boring to listen this word. But more specific how the system oscillation begins and what are the reason behind this oscillation? I also want to know why rotor oscillates? Is it occurs because of the change of main rotor field and revolving stator field? I want to know all the basics about the all above question.

Someone please Know me.
Thanks in advance.

 

For a system to oscillate, it must usually have two forms of energy storage and some means of transferring energy between these two forms. So in an electrical LC circuit, the two forms of energy are in the electrical field of the capacitor and the magnetic field of the inductor. With high voltage on the capacitor, most of the energy is in the electric field: with low voltage, there is a high current in the inductor and most of the energy is in the magnetic field.

In a pendulum, the two forms are gravitational potential energy and kinetic energy. At the top of the swing, all the energy is gravitational; at the bottom, the pendulum is moving at maximum velocity and the kinetic energy is maximum.

In an elastic system, the two forms are kinetic energy again, and the potential energy stored in the spring due to its compression or extension.

The alternator is a bit like the elastic system. However, it's complicated by the fact that the base state is not standstill but with the rotor moving in synchronism with the external alternating voltage. The rotor will have a fixed position which leads this voltage by an angle. There is a force which must be applied to maintain this lead angle, that depends on the sine of the angle.

So there is a balance in the steady state between the mechanical power into the rotor and the electrical power supplied to the electrical load. As long as this balance is maintained, the relative positions of the external voltage and the rotor will remain fixed. If there is a reduction in the electrical load, there is an excess of mechanical power which acts to increase the kinetic energy of the rotating parts - this increases the angle by which the rotor leads the external voltage.

The relative torque developed depends on the angle between the two, just as the force produced by a spring depends on the relative extension. As the rotor accelerates, the drag torque increases which reduces the torque available for acceleration. However, when the two come back into balance, the rotor is moving relative to the stator field and overshoots - so the imbalance goes the other way and the rotor speed is reduced again.
In fact, you can make a model of an alternator using two rotating assemblies coupled by a set of springs - I saw it illustrated years ago but can't remember the details. (I seem to recall Phil Corso, who posts here, may have some information).

The problem with the electrical machine is that the relative torque depends on the sine of the angle, an d reaches a maximum when the angle hits 90 degrees. Above this angle, the restoring torque starts to fall again and pole-slipping occurs.

More detailed analysis of this is available - google "alternator swing equation" - but gets quite mathematical. Hopefully, my description above will help.

 

Bruce...

The Mechanical Analogy you alluded to was devised by Prof S.B. Griscom in 1926. A description can be found in the Westinghouse Transmission and Distribution Reference Book, ca 1950! It's discussed in:

http://www.control.com/thread/1259307998

 

Dear Mr. Bruce Durdle,

As I am an Engineering student of this course then it is very difficult to understand the main fact of electromechanical oscillation. If you can answer my following question then it would be great for me. I am actually confused. Thanks in advance.

What do you mean by base state is not standstill? How can you briefly explain that the rotor have a fixed position and it leads the external alternating voltage? “There is a force which must be applied to maintain this lead angle, that depends on the sine of the angle”… what is this force and if there any equation relative to this line can you please explain me? And is this angle goes lagging sometime so we should provide that force to sustain this lead angle between rotor and voltage?

In your this post in 6th para what did you mean by two? Rotor accelerates but drag torque increases. what is this drag torque and where this drag torque come from? “However, when the two come back into balance, the rotor is moving relative to the stator field and overshoots”. I didn’t understand this line.

 

Thanks, Phil - I've had a quick look on the net for it and it doesn't appear to be available.

 

Let's try a simpler model first.

Think about someone water-ski-ing. In the usual case, the towing boat is connected to the skier by a line that is pretty well inelastic - this means that the skier will remain behind the boat by a fixed amount on a straight course. As the boat increases and decreases in speed, the skier will follow by the same distance and will be at the same speed as the boat at all times.

If the inelastic rope is replaced by an elastic bungee cord, the distance between the boat and skier will depend on the drag of the water acting on the skier. If the speed of the boat is held constant, the drag may increase or decrease depending on the angel of the sis to the water. Provided this does not change, the boat and skier will be in a fixed relationship to each other even though they are both moving at a high speed relative to the water.

Now, if the skier increases the angle of the skis slightly, the drag force will increase. To match this increase in the drag force, the bungee must stretch - and so the distance between the boat and the skier must increase. The process by which this happens is:

- In the steady condition, the pulling force applied to the skier from the cord is equal to the drag on the skier. The cord force is proportional to the extension of the cord.

- The drag increases. At this instant, the pulling force has not changed because the boat and skier are still the same distance apart. So the increased drag is not matched by the pulling force, and the skier starts to slow down.

- As the skier slows down, the distance between the boat and the skier increases, so increasing the stretch of the cord. This therefore increases the pulling force. The increase in pulling force reduces the overall force applied to the skier, and the rate of deceleration reduces.

- At some point, the drag and the pulling force will match and the skier will again be in equilibrium. However, his speed will be less than that of the boat and because of inertia effects the cord will lengthen beyond this point, making the pulling force exceed the drag and causing the skier to accelerate. When the skier's speed matches that of the boat, he will be further away from the boat than necessary to match the drag, and will speed up again to move towards the balance distance.

- At the balance distance this time, the skier is going faster than the boat and will again overshoot the distance.

This cycle will repeat but with the amplitude of the extension deviations decreasing each time until the system settles down at the new equilibrium position. new position is reached. Mathematically, it is exactly the same as any other cyclical process.

In a synchronous motor, the "speedboat" is replaced by the rotating electrical field developed by the three-phase stator winding. The "skier" is the magnetic field set up by the rotor windings. The rotor is attracted towards the stator with a rest position being when the two magnetic fields are aligned. With no load connected to the rotor, the rotor poles will be more or less in step with the rotating poles developed by the stator winding.

When a load is applied to the rotor, it puts a torque on to the rotor. To maintain the speed of the rotor, a torque is needed to pull it around. This torque comes from the magnetic attraction between the two fields, and this attraction is proportional to the angular separation between them for small separations - it is in fact proportional to the sine of the angle as I said. But for small deviations this nonlinearity can be ignored.

Now apply the process described for the boat and skier to the stator and rotor. At a steady load and speed, the stator field and rotor field will be moving at the same speed but with an angular separation between them that depends on the load torque. If the mechanical load is increased slightly, a new steady-state separation will apply, but to get there the rotor must slow down slightly, and will go through the same sequence of being too slow when the torques balance, the angle increasing to increase the pulling torque, accelerating again, and again overshooting the required position.

With an alternator, the same situation applies except that the "boat" is the rotor field, and the "skier" is the stator field.

So the base state is with the two elements moving at the same speed and separated by a fixed distance. In an alternator, the rotor poles will be more or less directly under the stator field poles when there is no load on the machine. In this condition, even though the two filed are closely aligned, there is no torque as the magnetic attraction is directed radially and has no component at right angles to the radius. If the mechanical power is increased slightly, the rotor will move ahead of the stator and there will be a small angle between the two poles. The attractive force will decrease slightly, but there will be a component of this force which is tangential to the rotor and will therefore develop some torque. As the angle increases, this tangential component will also increase.

This magnetic attraction between the two fields is what acts as an elastic element, and creates a torque dependent on the angle.

Have a look at http://ethesis.nitrkl.ac.in/1042/1/transient_stability_analysis_using_equal_area_criterion.pdf
- it may help. (You may need to remove any breaks added by the control.com system)

 

Bruce... if you want the Westinghouse T&D version contact me at:

[email protected]

Phil.

 

Dear Mr. Bruce Durdel & Mr.Phil Corso,

Recently I am troubling in understanding the fact on electromechanical oscillation. I only know that on steady state condition both rotor and stator revolving field rotates at the same speed. And they try to align themselves in the case of synchronous generator. So an electromagnetic torque creates which tries to oppose the rotor rotation and mechanical torque is given by primemover to sustain rotation.And there is a balance between input mechanical torque and output electrical torque.what happens when system is not steady state???

I also understand that when the load increase or decrease in grid then current passing through the generator stator winding also increase or decrease. So there is a change in stator revolving field relative to the value of current.Now the difference (difference can be angle, lead or lagging, field amplitude.......I don't know) between both of the field is not same as before on steady state situation.

Is this phenomena affects on electromechanical oscillation??? Or what should I learn more to understand this.

Please try not to give me other example of skiing, pendulum etc. because those difficult concept is really hard me to understand. If please give me something in the language of synchronous generator.

Thanks in advance.

 

Asif... since Bruce's Referenced Document also uses the SB Griscom Model, contact me for a copy!

Regards
Phil Corso [ [email protected] ]

 

Hi Asif,

> And there is a balance between input mechanical torque and output electrical torque. what happens when system is not steady state???

If the input mechanical torque and output electrical torque are not in balance, the difference is taken up by accelerating or decelerating the rotor. The complete force balance equation for the rotor is:

Input torque - (output torque + losses) = acceleration torque

The acceleration can be found by dividing the acceleration torque by the Moment of Inertia I. The losses are complex but have a component proportional to the speed difference between rotor and stator.

So if there is an imbalance between the driving torque and load torque, the rotor speed will start to change - very slowly. If the mechanical power is greater, the rotor will speed up slightly. This will lead to an increase in the angle between the rotor and stator, as seen from a viewpoint on the rotor itself (ie. a rotating frame of reference).

> I also understand that when the load increase or decrease in grid then current passing through the generator
> stator winding also increase or decrease. So there is a change in stator revolving field relative to the value of current.

When the load increases, the magnetic field developed by the stator will increase in magnitude. This field tends to oppose the stator field, so an increase will tend to reduce the net airgap flux. (This is the effect of armature reaction). The torque between the rotor and stator fields depends on the magnitude of this flux so will reduce, setting up an imbalance which allows the driven rotor to move slightly ahead as described above. As the angle between the rotor and stator fields increases, the tangential component of the magnetic force between them (which is the only part that affects the developed torque) will increase. This component is zero when the two fields are aligned, and increases as the angle increases - it depends on the sine of the angular difference.

> Is this phenomena affects on electromechanical oscillation??? Or what should I learn more to understand this.

Mechanical oscillations will occur whenever there is an accelerating force that increases as the magnitude of a displacement increases, acting against the direction of displacement and tending to reduce it, coupled with a system having inertia. In the alternator, the rotating parts have significant inertia, and the magnetic force between the rotor and stator increases as the angular difference increases, in a direction to pull them back together again. So the conditions for a mechanical oscillation are present. These conditions would be almost identical whether the rotor and stator were coupled magnetically, or through an elastic cord or spring.

> Please try not to give me other example of skiing, pendulum etc. because those difficult concept is really hard me to
> understand. If please give me something in the language of synchronous generator.

Complex situations such as the synchronous generator can often be understood better by relating them to something that is more familiar. My example of the speed-boat and skier was an attempt to describe how we can have oscillations about a moving "steady-state" situation - I'm sorry if it confused you.

Regards,
Bruce.

 

Here is my email address. You can send me one copy. Thanks in advance.

Regards
Asif Ferdoush [ [email protected] ]

 

Asif... repeating an earlier Caveat, "I don’t accept anonymous requests for information; something to do with the fact that anonymity breeds boorishness!"

Therefore, as a common courtesy I ask you provide your company affiliation, title or job function, and your location!

I will reciprocate with my qualifications and experience!

Regards,
Phil Corso (cepsicon [at] aol [dot] com)